Always prepend a space before kernel parameters.

It has been reported that in certain cases, qemud would think that
the serial device is /dev/ttyS1android.checkjni=1.

Change-Id: I977456ef23fb708b284cfe026da07ecce8d03bf1
diff --git a/vl-android.c b/vl-android.c
index 9407a82..ce9440e 100644
--- a/vl-android.c
+++ b/vl-android.c
@@ -5621,6 +5621,7 @@
         const char* kernel_parameters;
 
         if (android_hw->kernel_parameters) {
+            stralloc_add_c(kernel_params, ' ');
             stralloc_add_str(kernel_params, android_hw->kernel_parameters);
         }